Streptococcus sanguinis is a member of the viridans streptococci and is among the earliest colonizers of the dental plaque biofilm on the tooth surface. Within this niche, S. sanguinis is believed to play a beneficial role by competing with the growth of harmful bacteria in the mouth. However, S. sanguinis is also a leading cause of infective endocarditis, an often lethal infection of the cardiac valves. This research aims to identify genes that contribute to fitness in both the oral and cardiac environments. In this regard, we have recently begun to characterize the S. sanguinis com regulon. The most apparent function of this regulon is genetic competence- the ability to incorporate free, exogenous DNA from the environment. Based on previous studies in Streptococcus pneumoniae and Streptococcus mutans, we hypothesized that the com regulon might also be important for endocarditis virulence or biofilm formation. Our preliminary results suggest that the com regulon does not enhance endocarditis virulence;however it appears to have a significant effect on biofilm formation. Such findings suggest that this regulon may be an important determinant of competitiveness in the mouth. A model of the competence regulon in S. sanguinis is being constructed based on the well characterized system of Streptococcus pneumoniae. In S. pneumoniae, the competence inducing precursor peptide, ComC, is cleaved to its active form, competence stimulating peptide (CSP), and secreted to serve as a quorum sensing signal, inducing DNA uptake. Cleavage occurs at a conserved site through the activity of ComA, an ABC exporter with peptidase activity. Our results suggest S. sanguinis ComC is also processed and secreted as CSP. However, the conserved cleavage site is lacking in S. sanguinis ComC and no ComA ortholog or other protein capable of processing ComC has been identified. Despite our progress, many features of S. sanguinis com regulon induction are not understood. To further characterize such fundamental aspects of this important system with S. sanguinis we propose three aims: (1) to isolate and determine the sequence of secreted competence stimulating peptide (CSP) (2) to identify the S. sanguinis ComC export and processing system by screening a collection of defined mutants (3) to examine export and processing of S. sanguinis ComC in a heterologous host. Together, these aims provide a range of approaches for characterization of a system that may contribute in multiple ways to the competitiveness of S. sanguinis within the mouth. Once this system is better characterized, our findings may allow manipulation of the com system to enhance favorable properties of S. sanguinis in the mouth without enhancing its virulence for endocarditis. Public Health Relevance: The bacterium Streptococcus sanguinis is an important component of dental plaque and may benefit people by competing against harmful bacteria in the mouth. We are characterizing a system in Streptococcus sanguinis that enables it to take up DNA from its surroundings and may also help it survive in the mouth. This study proposes to characterize the most fundamental components of this important system.